A conventional infrared ray receiving circuit includes an infrared ray detector such as a PIN photodiode in general, an amplifier such as a differential amplifier, a band pass filter (defined as "BPF" hereinafter), a detector, and a wave-form shaping circuit.
In operation, an infrared ray LED is turned on and off in an infrared ray transmitting apparatus dependent on a pulse signal, so that a PPM signal of an infrared ray carrier wave having a specific wavelength is generated to be transmitted to the infrared ray receiving circuit and received by the infrared detector therein. The infrared ray detector detects the PPM signal to provide the amplifier with an optical to electric converted PPM signal. The amplifier amplifies the PPM signal with an adequate gain to be supplied to the BPF. The BPF which is tuned with the carrier wave of the PPM signal removes unnecessary signals and noises from the amplified PPM signal to supply the tuned PPM signal to the detector.
The detector detects the PPM signal having low and high durations to be supplied to the wave-form shaping circuit. The wave-form shaping circuit generates a pulse signal in accordance with the PPM signal having the low and high durations supplied from the detector, so that the pulse signal is supplied to an output terminal to be connected to a device such as a microcomputer, etc. In the generation of the pulse signal, an output signal of the detector which is a potential of a capacitor provided therein is compared with a hysteresis threshold level of a hysteresis comparator in the wave-form shaping circuit.
According to the conventional infrared ray receiving circuit, however, there is a disadvantage in that hair peaks are generated in the pulse signal which is the output signal of the hysteresis comparator, in a case where a difference of charge and discharge voltages of the capacitor provided in the detector is greater than a hysteresis width of the hysteresis comparator. On the other hand, when a capacitance of the capacitor is set to be larger to suppress the generation of the hair peaks, the output potential of the detector does not increase to be high sufficiently, if the PPM signal having a short ON duration is supplied thereto, so that the pulse width of the output signal of the hysteresis comparator becomes shorter than a prescribed width. The output signal of the hysteresis comparator is an output signal to be supplied to the output terminal as an output signal of the infrared ray receiving circuit. Therefore, the generation of the hair peaks in the output pulse signal or shortness of the pulse width thereof causes mulfunction of the device such as a microcomputer connected with the output terminal.